1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, such as perpendicular magnetic recording disks for use in magnetic recording hard disk drives, and more particularly to a continuous-media type of perpendicular magnetic recording disk with a granular cobalt-alloy recording layer having controlled grain size.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. One common type of perpendicular magnetic recording system uses a “dual-layer” media. This type of system is shown in FIG. 1 with a single write pole type of recording head. The dual-layer media includes a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits. Other proposed types of perpendicular magnetic recording systems use write-assist methods, such as thermal or heat assisted magnetic recording (TAMR or HAMR) and microwave-assisted magnetic recording (MAMR), and do not require media with a SUL.
The disk in FIG. 1 is a “continuous-media” disk wherein the RL is a continuous layer of granular cobalt-alloy magnetic material that becomes formed into concentric data tracks containing the magnetically recorded data bits when the write head writes on the magnetic material. A variation of a continuous-media disk is a “discrete-track-media” disk, meaning that the RL is patterned into concentric data tracks formed of continuous magnetic material, but the data tracks are radially separated from one another by concentric nonmagnetic guard bands. Continuous-media disks, to which the present invention is directed, are to be distinguished from “bit-patterned-media” (BPM) disks, which have been proposed to increase data density. In BPM disks, the magnetizable material on the disk is patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to continuous-media disks wherein a single “bit” may have multiple magnetic domains separated by domain walls.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording continuous-media disk showing the write field Hw acting on the recording layer RL. The disk also includes the hard disk substrate, a seed or onset layer (OL) for growth of the SUL, an intermediate layer (IL) between the SUL and the RL, and a protective overcoat (OC). The IL is a nonmagnetic layer or multilayer structure, also called an “exchange break layer” or EBL, that breaks the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and facilitates epitaxial growth of the RL. While not shown in FIG. 2, a seed layer (SL) is typically deposited directly on the SUL to facilitate the growth of the IL. As shown in FIG. 2, the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole (FIG. 1) which is the real write head (RWH) above the disk, and an effective secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the IL and by virtue of its high permeability produces a magnetic mirror image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field Hw inside the RL.
One type of material for the RL is a granular ferromagnetic cobalt (Co) alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL is achieved by the addition of oxides, including oxides of Si, Ta, Ti, and Nb. These oxides tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy form nonmagnetic intergranular material. A perpendicular magnetic recording medium with a RL of a CoPtCr granular alloy with added SiO2 is described by H. Uwazumi, et al., “CoPtCr—SiO2 Granular Media for High-Density Perpendicular Recording”, IEEE Transactions on Magnetics, Vol. 39, No. 4, July 2003, pp. 1914-1918. A perpendicular magnetic recording medium with a RL of a CoPt granular alloy with added Ta2O5 is described by T. Chiba et al., “Structure and magnetic properties of Co—Pt—Ta2O5 film for perpendicular magnetic recording media”, Journal of Magnetism and Magnetic Materials, Vol. 287, February 2005, pp. 167-171. As shown in FIG. 2, a capping layer (CP), such as a granular Co alloy without added oxides or with smaller amounts of oxides than the RL, is typically deposited on the RL to mediate the intergranular coupling of the grains of the RL.
The Co alloy RL has substantially out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of its hcp crystalline structure being induced to grow substantially perpendicular to the plane of the layer during deposition. To induce this growth of the hcp RL, the IL onto which the RL is formed is also an hcp material. Ruthenium (Ru) and certain Ru alloys, such as RuCr, are nonmagnetic hcp materials that are used for the IL.
The enhancement of segregation of the magnetic grains in the RL by the additive oxides is important for achieving high areal density and recording performance. The intergranular oxide material not only decouples intergranular exchange but also exerts control on the size and distribution of the magnetic grains in the RL. Current disk fabrication methods achieve this segregated RL by growing the RL on a Ru or Ru-alloy IL that exhibits columnar growth of the Ru or Ru-alloy grains. The columnar growth of the IL is accomplished by sputter depositing it at a relatively high sputtering pressure. FIG. 3 is a transmission electron microscopy (TEM) image of a portion of the surface of a CoPtCr—SiO2 RL formed on a Ru IL. FIG. 3 shows well-segregated CoPtCr magnetic grains separated by intergranular SiO2. However, as is apparent from FIG. 3, there is a relatively wide variation in the size of the magnetic grains. A large grain size distribution is undesirable because it results in a variation in magnetic recording properties across the disk and because some of the smaller grains can be thermally unstable, resulting in loss of data.
What is needed is a continuous-media perpendicular magnetic recording disk that has a granular cobalt alloy RL with additive oxides with well-segregated magnetic grains of substantially the same size, i.e., minimal grain size distribution.